| Based on the green environmental protection, easy to realize automation and excellent performance of the joint, friction stir welding (FSW) has been paid attention by aerospace field since it was invented in 1991. The rotation speed was set at 1000 r/min, and welding speed was set at 80 mm/min, microstructure and properties of friction stir butt welding joint of dissimilar domestic aviation aluminum alloy 2A12-T4 and 7075-T6 were firstly studied in the paper, both the sheets measured 5 mm thickness. X-ray diffraction (XRD) and metallographic microscope were adopted to study the microstructure in each area of the obtained FSW joint. Properties of the 2A12/7075 FSW joint and base materials were investigated by microhardness test, tensile test, impact toughness test, fatigue test and intercrystalline corrosion test. In addition, fracture morphologies of FSW joint and base materials were observed and studied by stereo microscope, scanning electron microscope (SEM), and energy dispersive spectrometer (EDS). Moreover, preliminary exploration of the fracture mechanism has been implemented.XRD showed that the matrix of 2A12-T4 alloy was ?(Al), strengthening phases were S(Al2CuMg) and ?(Al2Cu), and impurity phase was Mg2Si. Matrix of 7075-T6 alloy was also a(Al), strengthening phase was ?(MgZn2), but it contained many impurity phases such as Q(Al1.9CuMg4.1Si3.3) and Al0.7Fe3Si0.3. Matrix of the FSW joint was still a(Al), and the strengthening phases were S(Al2CuMg), T(AlMg4Zn11) and ?(AlCu3). The study of microstructure showed that 7075-T6 aluminum alloy was composed of the distorted lath structure. Besides the distorted lath structure, there was a little recrystallization structure in 2A12-T4 aluminum alloy. High temperature produced by friction heat and strong plastic deformation resulted in a fully dynamic recrystallization in the nugget zone (NZ). NZ was composed of uniform distributed equiaxed grain structure, and the grain size was about 5.7 ?m. Under the effect of welding thermal cycle, grains in heat affected zones (HAZ) at two sides grew at some extent. Inadequate mechanical mixing effect in thermo-mechanical affected zone (TMAZ) lead to the distinctly distorted lath structure in TMAZ at advanced side (A-TMAZ), and lightly elongated equiaxed grain structure in TMAZ at retreated side (R-TMAZ).Vicker's hardness test indicated that, the microhardness of the 2A12/7075 FSW joint approximately presented a "W" distribution. Hardness of 7075-T6 and 2A12-T4 base materials were 154.1 and 138.0 respectively. Hardness value in HAZ at the two sides both decreased in some degree, especially in the HAZ at advanced side which was only 128.0. The highest hardness value in welding area appeared near the center of joint, which reached 146.5. Research on tensile property showed that, the tensile strength of 7075-T6,2A12-T4 and FSW joint were 551 MPa,464 MPa, and 392 MPa respectively; and elongation rate of them were 8.6%,13.5%, and 5.8% respectively. All the tested joint specimens cracked at the HAZ of advanced side, tensile property of the joint decreased some extent compared to the base materials. Impact toughness tests indicated that, the impact energy of NZ was higher than that of two base materials, and HAZ in advanced side performed the lowest impact energy. Fatigue tests showed that the fatigue strength of 2A12/7075 FSW joint,7075-T6 and 2A12-T4 were 101 MPa,113 MPa and 117 MPa respectively. Considering the trend of S-N curve, fatigue strength, K and simulated result of S-N curve, fatigue property of the 2A12/7075 FSW joint was not good as that of 2A12-T4 base material, but better than that of 7075-T6 base material. Intergranular corrosion tests indicated that, the intergranular corrosion resistance of the joint was in decreasing order in NZ, R-TMAZ,2A12, R-HAZ, A-TMAZ, A-HAZ, and 7075. Generally, intergranular corrosion resistance of retreated side was better than that of advanced side.Fracture analysis showed that, mechanism of tensile rupture of 7075-T6 base material was quasi cleavage fracture, impact rupture was cleavage fracture with a tongue pattern, fatigue rupture at low cycle was mainly cleavage fracture, and fatigue rupture at high cycle was intergranular fracture. Mechanism of tensile rupture of 2A12-T4 base material was a ductile fracture with shallow dimples, impact rupture process was a brittle fracture with herringbone patterns, fatigue rupture at low cycle was mainly quasi cleavage, and fatigue rupture at high cycle was a ductile and brittle mixed fracture but mainly equal-axis dimples. Mechanism of tensile rupture of FSW joint was cleavage fracture, impact rupture was an intergranular fracture, fatigue rupture at low cycle was quasi cleavage, and fatigue rupture at high cycle was a ductile and brittle mixed fracture mainly intergranular fracture. |
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